1. Field of the Invention
The present invention relates to an exhaust gas purifying system for use in an automobile and in particular to an exhaust gas purifying system for temporarily adsorbing an automobile exhaust gas by an adsorbent until an exhaust gas treating catalyst becomes active.
2. Prior Art
The exhaust gas emitted from automobiles are one of the main causes of air pollution and therefore are regulated by a number of regulations on exhaust gas emissions.
The current exhaust gas emission level regulations are complied with by the treatment of the exhaust gas with a ternary catalyst including platinum, rhodium, etc. The unburnt hydrocarbons and nitrogen oxides and the like in the exhaust gas emitted from an automobile have been considerably decreased compared to emissions prior to these regulations.
However, in the prior art, the exhaust gas can not be treated with the ternary catalyst until the temperature of the ternary catalyst is elevated to a temperature at which it becomes active (the light off temperature, about 350.degree. C.). The unburnt hydrocarbons are discharged without being treated particularly immediately after starting of the engine, although the concentration of the unburnt hydrocarbons is very high. If the starting temperature is, for example, 20.degree. C., about 100 seconds are taken for the catalyst to reach the light off temperature (about 350.degree. C.). In this period of time, unburnt hydrocarbons having a very high concentration which can be as high as 7000 to 8000 ppm are discharged.
It has been proposed as a countermeasure to this problem to preliminarily heat a catalyst with an electric heater as is known by SAE paper 900503 and Japanese Unexamined Patent Publication No. Tokkai-Hei 3-31510. These proposals have a practical difficulty since it requires such a large amount of electric power.
It has been known that unburnt hydrocarbons are adsorbed by an adsorbent upstream of a catalyst until the catalyst is activated (Japanese Unexamined Patent Publication No. Tokkai-Sho 63-68713).
Recently, air pollution has become a more serious concern again, since the absolute amount of the emitted exhaust gas has increased due to the increase in the number and the size of automobiles. It has been decided that stricter exhaust gas regulations will be enforced as a solution for this air pollution problem. For example, it has been decided that LEV (Low Emission Vehicle) regulation will be enforced in California, U.S.A. from 1997. This regulation will be enforced in all States in the U.S.A.